1. Field of the Invention
The present invention relates to an image reading apparatus, and more particularly to a sheet-through type image reading apparatus that is suited to be employed in a copying machine or a scanner as an image input device.
2. Description of Related Art
In the art of image reading apparatuses for reading images optically, a platen-set method wherein an image of a document set on a platen glass is read and a sheet-through method wherein an image of a document is read while the document is being fed are adopted separately or at the same time. The sheet-through method is advantageous in its small size, low cost, low noise, high-speed image reading and high printing efficiency. Therefore, the sheet-through method is generally adopted for monochromatic copying machines and color copying machines.
In the sheet-through method, an image reading position is fixed on a transparent member (a strip of reading glass), and a reading optical system is designed to have a focus on an imaged-surface of a document fed over the reading glass. Therefore, image reading is apt to be influenced by foreign matters such as dust adhering to the reading glass, and portions shaded by the foreign matters are read as image noise, that is, stripes. When the document is paper, there is inevitably raised a problem that very small foreign matters such as loading material (e.g., calcium carbonate) and fiber contained in the paper adhere to the reading glass.
In order to avoid this trouble, conventionally, various measures as follows have been taken during the step of processing the read image; detecting noise in the read image and alarming the user; erasing stripes in the step of image processing; or moving the reading glass not to read the dust any further. However, none of these measures can solve the problem that dust is deposited on the reading glass, which consequently necessitates a serviceman to clean the reading glass.
Japanese Patent Laid-Open Publication No. 2000-270152 discloses an image reading apparatus comprising an elastic cleaner that cleans a reading glass while rotating. Since foreign matters are carried onto the reading glass by documents fed thereto, it is preferred that the cleaning is performed at intervals among arrivals of documents. The use of a rotary cleaner requires only a short time for cleaning, and therefore, this method is advantageous to the image reading efficiency.
In order to improve the image reading efficiency, the intervals among documents fed to the reading position shall be minimized. However, the minimization of the intervals among documents shortens the time for which the reading glass can be exposed to the cleaning, and satisfactory cleaning of the reading glass becomes impossible. Further, there are various image reading modes, such as a high-resolution mode, a low-resolution mode, etc., and the cleaning operation must be varied in accordance with the various image reading modes.
Thus, the image reading efficiency and the cleaning effect are incompatible with each other, and it is necessary to adjust the cleaning operation to the document feeding speed, the intervals among documents, etc. However, Japanese Patent Laid-Open Publication No. 2000-270152 does not discuss this point.